Prions are the basis of several fatal neurodegenerative disorders such as Creutzfeldt-Jakob disease in humans, mad cow disease in cattle, and scrapie in sheep. In yeast, prions have been found to underlie several non-Mendelian phenotypes. Despite differences in sequence, yeast prions share similar features with human prions including infectivity, prion strain phenomenon, and species barrier. Therefore, yeast prions are excellent model systems to study the mechanism of prion diseases. Several key aspects of human prion diseases are unambiguously illustrated using yeast prions, including the protein only hypothesis, conformational variations as the basis of prion strains, and involvement of chaperones. A knowledge gap in the prion field is the lack of detailed high-resolution structures for prion fibrils. In this project, we aim to determine fibril structures of he yeast prion protein Ure2, one of the best studied yeast prions, under quiescent and agitated conditions. It has been proposed that different fibril structures are the basis of different prion strains. Our preliminary studies have shown that Ure2 fibrils indeed adopt different structures under quiescent and agitated conditions. Full- atom structural models of Ure2 fibrils under these two conditions will bring insights into the structural basis of prion strains and mechanism of prio propagation. This project consists of three specific aims. In Aim 1, we will determine the ?-strand and turn/loop regions in Ure2 fibril under quiescent and agitated conditions. In Aim 2, we will obtain an extensive set of inter-residue distance constraints for quiescent and agitated Ure2 fibrils. In Aim 3, we will use the experimental constraints and structure prediction program Rosetta to calculate atomic- level structure models for quiescent and agitated Ure2 fibrils.